1. Field of the Invention
The present invention relates to a control system for controlling the engagement force of an electromagnetic clutch at a target engagement force by controlling the current supplied to its coil.
2. Description of the Prior Art
There is a known technique for enhancing turning performance of a vehicle where an engine driving force can be distributed between right and left driven wheels via a driving force distribution system that includes two clutches, and the driving force distributed to the outer turning wheel is increased while the driving force distributed to the inner turning wheel is decreased so as to generate a yaw moment in the turning direction. In general, in an arrangement in which the two clutches are electromagnetic clutches, target magnetic flux densities are calculated from target engagement forces for the electromagnetic clutches. The target excitation currents that are supplied to coils of the electromagnetic clutches are calculated from the target magnetic flux densities, and feedback control is carried out so that actual excitation currents flowing through the coils agree with the target excitation currents.
In the above-mentioned arrangement, the target excitation current is calculated from the target magnetic flux density assuming that an air gap of the armature of each of the electromagnetic clutches is a constant size. However, when the air gap varies due to the state of wear of frictional engagement members of the electromagnetic clutch and the variation in the position of the armature when the electromagnetic clutch is in a disengaged state, an error is caused in the relationship between the target magnetic flux density and the target excitation current, and a proper target excitation current cannot be calculated, thereby raising the possibility that the control precision might be degraded.
The present applicant proposed in Japanese Patent Application No. 2000-219055 that degradation in the responsiveness of engagement of an electromagnetic clutch could be prevented by calculating a delayed target magnetic flux density from a target magnetic flux density of the electromagnetic clutch while taking into consideration a delay factor in the control system; calculating a deviation between the percentage change in the delayed target magnetic flux density obtained by differentiating the delayed target magnetic flux density with respect to time and the percentage change in the actual magnetic flux density of the electromagnetic clutch; and compensating, based on the deviation, for an error in the target excitation current caused, by external perturbations such as a widened air gap of the armature, at the instant the coil is excited.
However, since the arrangement proposed in Japanese Patent Application No. 2000-219055 requires means for calculating the percentage change in the actual magnetic flux density, means for calculating the delay factor, differentiating means, means for calculating the deviation, means for gain multiplying, etc., there is the problem that the structure of the control system becomes complicated.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to allow the engagement force of an electromagnetic clutch to be precisely controlled at a target engagement force by a simple structure even when the air gap of the armature of the electromagnetic clutch varies.
In order to achieve the above-mentioned object, in accordance with the invention, an electromagnetic clutch control system is provided that includes a magnetic flux density sensor for detecting an actual magnetic flux density that flows in an electromagnetic clutch, target engagement force calculating means for calculating a target engagement force of the electromagnetic clutch, target magnetic flux density calculating means for calculating a target magnetic flux density of the electromagnetic clutch based on the target engagement force calculated by the target engagement force calculating means, and feedback control means for feedback control of the current supplied to the electromagnetic clutch so that the actual magnetic flux density agrees with the target magnetic flux density.
In accordance with the above-mentioned arrangement, since the target magnetic flux density of the electromagnetic clutch is calculated based on the target engagement force of the electromagnetic clutch, and the current supplied to the electromagnetic clutch is feedback controlled so that the actual magnetic flux density flowing through the electromagnetic clutch agrees with the target magnetic flux density, even when the air gap varies due to wear, etc. of the frictional engagement members and the relationship between the actual magnetic flux density and the current changes, the engagement force of the electromagnetic clutch can be precisely controlled at the target engagement force.